Welcome to HWC!. Consider adding a SSD for your OS. If you decide to get more storage, aka 1tb drive, make sure it's a new gen. sata 3. Much faster.

Your motherboard doesn't allow more than one graphics card, so adding another isn't an option. In order to get better performance from your graphics card, you would need to get something more powerful, like the 470 or 480.

Find out the make of your psu b4 doing this. Some psus can handle higher demands easily while others won't. The power supply is one of the most important components in your system, but you don't have to get one that's way more than what you need. Some leeway is good.

If you make a lot of dvds or copy a lot of discs, another dvd RW would be good. Otherwise, it should be ok.

Honestly there is nothing you have to upgrade. Seems like a pretty good build, but I don't know exactly what parts are in there. If gaming performance is good enough for you, then the only suggestions I have are for an SSD(not a requirement, but will be noticeable in everyday use), and make sure the power supply is decent.

It's a matter of personal preference. I recommend only changing your system if you are unhappy with your current system.
So is there anything you'd like your system to be able to do that it doesn't do now or anything you are unhappy with?